Electromagnetic interference ("EMI") is an electrical noise current which is usually present in the radio-wave frequency range. This current originates from within a system of electrical devices rather than an outside source such as a radio signal transmitter. Electric power supplies are known to be significant generators of EMI. In connection with electric power supplies, EMI is undesirable because, for example, it can disturb the operation or degrade the performance of other equipment connected to the same source of AC power.
EMI filters frequently incorporate "Y" capacitors ("Y-caps") as part of their noise suppression circuitry. These circuit elements typically connect near a power supply's input terminals and terminate at an electrical ground. By diverting common-mode current to an electrical ground, undesirable electrical noise current can be suppressed thereby preventing EMI from leaving the power supply via its input terminals and disturbing other equipment electrically connected to the same AC source.
In most known power supplies, EMI filtering incorporates a Y-cap circuit which is permanently connected to the power supply ground within the power supply's housing. This configuration typically allows the capacitor to be located very close to the electrical ground, thereby optimizing EMI suppression. However, the internally mounted Y-caps of conventional power supply devices are very difficult to disconnect from the power supply ground.
For a variety of reasons, some power supply end-users do not want a Y-cap circuit permanently connected to the power supply ground. One drawback in using a Y-cap circuit for EMI suppression is the attendant "leakage current" which flows into the electrical ground. For instance, whenever an alternating current (AC) voltage is applied across a Y-cap circuit, some amount of current will "leak" through it.
Some power supply users prefer a supply with minimal or no leakage current, even at the expense of reduced EMI suppression. For example, electric power supplies are frequently used to drive medical equipment. In this application of a power supply, leakage current must be reduced in order to ensure the safety of medical patients and equipment operators.
It is therefore an object of the present invention to provide a power supply device which has externally detachable "Y" capacitors located within the power supply.
It is a further object of the present invention to provide a power supply with an EMI suppression circuit which has a low-impedance path to an electrical ground.